GaN-family nitride semiconductor devices such as light-emitting diodes and semiconductor lasers are conventionally produced by growing a multi-layered GaN-family compound semiconductor layer on a template for epitaxial growth (for example, see Non-patent Document 1). FIG. 7 illustrates a typical crystal layer structure of a conventional GaN-family light-emitting diode. The light-emitting diode shown in FIG. 7 has a laminated structure. In the structure, after an underlying layer 102 of AlN is formed on a sapphire substrate 101 and then a cyclic groove pattern is formed thereon by photolithography and reactive ion etching, an ELO-AlN layer 103 is formed; plus an n-type clad layer 104 of n-type AlGaN having a thickness of 2 μm, an AlGaN/GaN multi-quantum well active layer 105, a p-type AlGaN electron block layer 106 having a higher Al composition ratio than the multi-quantum well active layer 105 and having a thickness of 20 nm, a p-type clad layer 107 of p-type AlGaN having a thickness of 50 nm, and a p-type GaN contact layer 108 having a thickness of 20 nm are sequentially stacked on the ELO-AlN layer 103. The multi-quantum well active layer 105 has a multi-layered structure including five stacking layers including a GaN well layer of 2 nm in thickness that is sandwiched with AlGaN barrier layers of 8 nm in thickness. After crystal growth, in order to expose a portion of the surface of the n-type clad layer 104, the multi-quantum well active layer 105, the electron block layer 106, the p-type clad layer 107, and the contact layer 108 thereon are etched off. A p-electrode 109 of Ni/Au is formed on the surface of the contact layer 108, for example, and an n-electrode 110 of Ti/Al/Ti/Au is formed on the surface of the exposed n-type clad layer 104, for example. By making a GaN well layer into an AlGaN well layer and changing the Al composition ratio or the thickness of the AlGaN well layer, the emission wavelength is shortened, or by adding In to the layer, the emission wavelength is lengthened, thus providing a light-emitting diode in an ultraviolet region having a wavelength of about 200 nm to 400 nm. Semiconductor lasers may be produced similarly. In the crystal layer structure shown in FIG. 7, a template for epitaxial growth is formed of the sapphire substrate 101, the AlN underlying layer 102, and the ELO-AlN layer 103.
The crystal quality of the template surface directly affects the crystal quality of the GaN-family compound semiconductor layer formed thereon, providing significant effects on the characteristics of a light-emitting device or the like which is formed as a result. In providing a light-emitting diode or a semiconductor laser in the ultraviolet region, it is particularly desirable to use a template having a reduced threading dislocation density of 107/cm2 or less or preferably about 106/cm2. When the ELO-AlN layer 103 is epitaxially grown by an epitaxial lateral overgrowth (ELO) method on the AlN underlying layer 102 having a cyclic groove pattern as shown in FIG. 7, the AlN layer grown from the flat surfaces of the protrusions between the grooves overgrows laterally so as to cover over the tops of the grooves, and at the same time, the threading dislocation grown from the flat surfaces concentrates above the grooves due to the lateral overgrowth, so that the threading dislocation density is reduced significantly.
However, for the template including the sapphire substrate, the AlN underlying layer, and the ELO-AlN layer as shown in FIG. 7, it is necessary to remove a sample (substrate) once from a reaction chamber for an epitaxial growth after the AlN underlying layer is grown, and to form a cyclic groove pattern on the surface of the AlN underlying layer by photolithography and reactive ion etching. Thus, the AlN underlying layer and the ELO-AlN layer cannot be grown continuously, and the manufacturing process becomes complex and the throughput decreases, thereby increasing manufacturing costs.
On the other hand, in order to prevent the complication of the manufacturing process and the decrease in throughput by omitting etching between crystal growth processes, proposed are the methods of providing a template for epitaxial growth in which a cyclic groove pattern is directly formed on the surface of the sapphire substrate by photolithography and reactive ion etching or the like, and the ELO-AlN layer is formed directly on the sapphire substrate (for example, see Patent Document 1, and Non-patent Documents 2 and 3). In order to grow an ELO-AlN layer on the grooved surface of a substrate, it is preferable to form deeper grooves on the sapphire substrate surface since the AlN layer grown from the bottom of the grooves needs to be separate from the AlN layer laterally overgrown from the flat surfaces of the protrusions between the grooves. However, the sapphire substrate has a low etching rate and is difficult to process, so that an ELO-AlN layer having a low threading dislocation density needs to be grown on a shallow-grooved substrate.
When an AlN layer is grown on a sapphire substrate without using a lateral overgrowth method, problems solved by using the lateral overgrowth method become apparent. Thus, it is very difficult to produce a template having no cracks, a reduced threading dislocation density, and a good morphology of crystal surface.
As a method for producing a template having no cracks, a reduced threading dislocation density, and a good morphology of crystal surface by growing an AlN layer on a sapphire substrate without using a lateral overgrowth method, for example, there is suggested a method for forming a multi-layered structure by alternately laminating a pulse flow AlN layer grown by continuously supplying TMA (trimethylaluminum) as a material of Al and intermittently supplying NH3 (ammonia) as a material of N and a continuous AlN layer grown by continuously supplying TMAl and NH3 (see Non-patent Document 4).